The present invention relates to generators and, more particularly, to apparatus for monitoring current in internal armature connections of a large, three-phase generator.
Although the present invention may have applicability to other types of generators, for purposes of concreteness of description, the following disclosure is directed toward a two-pole, three-phase generator.
A large two-pole, three-phase generator conventionally includes a laminated stator having longitudinal slots into which conducting bars are installed. The ends of the conducting bars are interconnected by end turns to produce three stator windings whose centers are physically and electrically spaced apart by about 120 degrees. Each of the three stator windings is made up of two phase belts which may be connected in series or parallel to form windings referred to as 1-circuit or 2-circuit windings respectively. Both ends of each winding are brought out of the generator stator frame to terminals for connection to external circuits. Each of the six terminals is encircled by one or more large, iron-core, current transformers for detecting problems in the stator or its load.
External connection of the terminals may be arranged to produce either a wye-connected or a delta-connected generator. In practice, however, the delta connection is rarely used for utility power generation. A wye connection is conveniently formed by placing a short-circuit jumper across the appropriate end of each of the windings as dictated by the desired phase relationships therebetween. This shorted end of each winding provides a neutral point at which the windings can be grounded. It is thus conventional to identify the three terminals which are to be jumpered together and grounded as the neutral terminals. The remaining three terminals are conventionally called the high-voltage terminals.
Certain test operations such as, for example, periodic high-voltage testing of individual windings, require that provision be made for isolating each phase by removing the jumper connecting the neutral ends of the three windings.
A substantial amount of space is required for mounting the three high-voltage terminals and the three neutral terminals, and for accommodating the internal connections. The six terminals must be spaced apart sufficiently to accommodate the large physical size of conventional iron core current transformers provided on each terminal for measuring the current in the respective windings. To satisfy the internal and external space requirements for connection and interference avoidance, it is conventional to provide a large lower frame extension which extends below the envelope of the generator stator frame.
The maximum permissible envelope size of a generator is established by clearance requirements of curves, bridges and tunnels between the point of manufacture and the point of installation. If the lower frame extension is integrally installed during manufacture of the generator, both the generator envelope dimension and its generating capacity would be reduced. It is thus conventional to ship the lower frame extension separately from the generator and install it at the destination.
A lower frame extension is a large, expensive structure weighing, for example, several tons. Its on-site installation adds significantly to the labor to place a generator in operation. The elimination of the lower frame extension is thus desirable.
The neutral terminals are shorted together in a neutral enclosure external to the generator stator frame. Such a neutral enclosure may be a stainless steel box as large as about four feet by four feet by eight feet. Both the neutral terminals, and their associated enclosure, are expensive and their elimination is also desirable.
Some generators have been built in which the neutral terminals, and their iron core current transformers, have been moved from the lower frame extension to positions within the generator wrapper. Such a change can only be performed when sufficient internal space within the generator wrapper is available for containing the connections and current transformers sufficiently spaced apart to avoid mutual interference. This is not feasible in most generator designs.